1. Field of the Invention
The invention relates to the field of laser therapy, and in particular, to the thermal treatment of biological tissues with laser radiation.
2. Description of the Prior Art
Laser induced selective photothermolysis is currently the preferred technique in treatment of many vascular disorders of the dermis. Typical examples are treatment of port wine stains and telangiectasia with short laser pulses in the green/yellow wavelength region. This light is strongly absorbed in hemoglobin whereas the absorption in other dermal constituents such as proteins and lipids are quite insignificant. The laser pulse is selected to be sufficiently long to allow heat to diffuse from the lumen to the entire vessel wall, but short enough to prevent thermal damage to perivascular structures.
Typical lasers currently being used in the clinic are flashlamp pumped dye lasers emitting at 585 nm wavelength and second-harmonic neodymium doped yttrium-aluminum-garnet lasers (Nd:YAG) at 532 nm. The pulse lengths are in the range of 0.5-1.5 ms.
However, thermal damage to the epidermis represents a severe problem that limits the acceptable power density of the laser beam. This is a general problem for laser therapy, because although the melanin absorption decays strongly with increasing wavelength, the absorption is significant over the entire visible and near-infrared part of the optical spectrum. Damage occurs when the epidermal temperature rises to about 70xc2x0 C., i.e., about 35xc2x0 C. above the normal temperature of 32-34xc2x0 C.
These limitations can be significantly reduced if the epidermis is selectively precooled well below the normal temperature. Cooling of the epidermis to about 0xc2x0 C. will allow for a 70xc2x0 C. temperature rise, which corresponds to an increase in the maximum acceptable power density by a factor of about two. The maximum acceptable power density for a 585 nm laser used without cooling is in the range of 6-8 j/cm2, whereas 15 J/cm2 is routinely used with cooling at Beckman Laser Institute and Medical Clinic, such as disclosed in Nelson, et al., xe2x80x9cApparatus And Method For Dynamic Cooling Of Biological Tissues For Thermal Mediated Surgery,xe2x80x9d U.S. Pat. No. 5,814,040 (1998), which is incorporated herein by reference.
The invention is a method of controlling temperature profiles of tissue subjected to cycles of cooling and heating comprising the steps of cooling the tissue and heating the tissue. The step of cooling the tissue is controlled according to a heat transfer coefficient referred to a tissue junction relative to a surface of the tissue subjected to cooling.
In the illustrated embodiment the tissue is skin having a stratum corneum and epidermis. However, it must be understood that any tissue, human or animal, is contemplated as being with the scope of the invention. The tissue junction in the case of the illustrated embodiment is an interface between the stratum corneum and epidermis. The step of cooling the tissue comprises the step of controlling cooling of the tissue according to an effective heat transfer coefficient between a cooling agent at the surface of the skin and the interface between the stratum corneum and epidermis.
The step of cooling is controllable by an amount of cooling applied to the tissue. The step of cooling the tissue according to the heat transfer coefficient comprises the step of varying the amount of cooling according to distance from the surface of the tissue to the tissue junction. In this way, for example, the cooling applied to various skin lesions is customized to the thickness of the lesion being treated. In particular in the illustrated embodiment, the step of cooling the skin according to the heat transfer coefficient comprises the step of varying the amount of cooling according to distance from the surface of the skin to the interface between the stratum corneum and epidermis.
The step of cooling the tissue is controlled according to a heat transfer coefficient is based on the step of determining the heat transfer coefficient using a thermally conductive metal target mounted in an insulating substrate as a standard which is exposed to the cooling.
In the illustrated embodiment the step of cooling comprises spraying cryogen droplets onto the tissue. However, any agent for cooling, now known or later devised, may be employed and the invention is not limited to cryogenic cooling.
The step of cooling the tissue is controlled according to a heat transfer coefficient is based on determining the heat transfer coefficient using a thermally conductive metal target mounted in an insulating substrate as a standard which is exposed to the cooling.
The invention is also characterized as an apparatus for performing the above method and the standard by which the heat coefficient is determined.
While the method has been described for the sake of grammatical fluidity as steps, it is to be expressly understood that the claims are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations under 35 USC 112, but to be accorded the full scope of the meaning and equivalents of the definition provided by the claims. However, it is intended that the scope of the invention never be less than the scope of equivalency provided by 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.